The generation of jets is associated with two possible causes: linear and non-linear. The first one is the discreteness of the wave vector lattice in the pumping region, \(1 \le k \le 6\) , which inevitably leads to a small anisotropy in the excitation and differences in the growth of perturbations at the initial stage.
The second one is the tendency of the dispersion to zero at small \(k\)  caused by the equations itself; the three-wave resonance conditions are satisfied for an arbitrary beam. The beams that form the jets have an advantage over other beams. This process, the cooperation of rays into a jet, has a clearly nonlinear character. In particular, this fact follows from the numerical simulation results presented in Fig. REF  and Fig. REF , for which the contrasts in intensity in the jets and the regions between them are significant: the difference reaches two orders of magnitude. In our opinion, such a jump in intensity can not be explained only by a small anisotropy in pumping, but has a nonlinear origin, possibly due to the collapse of sound waves described by the three-dimensional Kadomtsev-Petviashvili equation (see review [1]}). It should also be noted that numerical experiments show that similar jets appear at the onset of developed hydrodynamic turbulence [2]}. However, this question is beyond the scope of this work: new numerical experiments with a high spatial resolution are required.
<FIGURE>